1. Field of the Invention
The present invention relates to a viewing device for X-ray and like films, and more particularly to an improved viewing device of the type described.
2. The Prior Art
X-ray films are conventionally inspected by a physician or radiologist by disposing the same against a diffuser plate behind which is located a lighting source. X-ray films are supplied in a plurality of standard sizes. To accommodate the range of X-ray film sizes conventional devices include a diffuser plate substantially larger than the largest films likely to be viewed. When an X-ray plate mounted on such viewing device is inspected, a substantial glare is encountered as a result of the light passing through such portions of the diffuser out of registry with the X-ray film. A result of the glare condition noted involves significant eye strain on the part of the viewer required to inspect such plates over protracted periods. Moreover, a person reading an X-ray and exposed to the high light concentrations in areas directly surrounding the film will be less able to perceive critical but minor shadings and nuances in the film by reason of the contraction of the iris of the eye resulting from the high light concentrations passing the diffuser plate in areas surrounding the film.
While it is of course feasible for a viewer to overlay masking strips on the diffuser plate and thus block unwanted and contrast reducing light passing through the diffuser, as a practical matter readers of X-rays rarely resort to such practice.
Attempts have been made in the past to provide X-ray film reviewing devices which shield the eyes of the viewer from light emanating from the light source other than the light passing through the film.
In U.S. Pat. No. 1,988,654, Haag, there is disclosed a light box which incorporates two manually movable curtains for masking all of the light transmitting surfaces of a diffuser up to the edges of the film.
U.S. Pat. No. 2,436,162, Cadenas, discloses an X-ray viewer having a masking arrangement incorporating a plurality of hingedly connected opaque masks which may be manually pivoted relative to each other to expose all or only selected parts of an X-ray film.
U.S. Pat. No. 4,004,360, Hammond, is directed to a self-masking viewing device which purports automatically to obscure areas of the viewing screen not occupied by the X-ray film.
In accordance with such device, the screen is provided with a multiplicity of holes, which holes may be selectively blocked by shutters or opened for the passage of light. The interior of the device is connected to a vacuum source which functions to hold the film against the front surface of the device.
The vacuum functions, in addition, to close the shutters connected with those of the holes not covered by the film thereby passage of light through such holes is prevented. Air cannot pass through those of the holes in registry with the film and, thus, the shutters associated with the film covered holes remain open for the passage of light.
The device described is unsuitable for critical X-ray film inspection since the presence of holes and shutters in the viewing screen in the areas in registry with the film impose a pattern behind the film which interferes with the accurate reading of the film.
U.S. Pat. No. 4,373,280, Armfield, discloses an X-ray viewer having a cross bar for supporting films at a central portion of the screen. A series of shades is provided which may be manually activated to obscure selected parts of the illuminated surface not occupied by the film.
From the foregoing description of the most relevant prior art known to applicant it will be perceived that there has not heretofore been available an apparatus capable of use with X-ray films of a variety of sizes which automatically masks the areas not occupied by the film and which does not interpose between the light source and the film inspected a background or pattern which interferes with a critical examination of the film.